1. Field of the Invention
The present invention relates to an electrostatic image forming toner and developer used in image forming apparatuses such as copiers, printers and facsimiles.
2. Description of the Related Art
With the recent trend toward environmentally sensitive products, techniques of fixing toner with low energy have been demanded.
There are various kinds of such techniques but among others, stronger demand has arisen for electrostatic image forming toner at lower temperatures.
A common method for lowering the fixing temperature of toner is lowering the glass transition temperature of its toner binder. However, simply lowering the glass transition temperature easily causes aggregation of powder (blocking). Aggregation of toner particles in an image forming apparatus impairs the operation of its developing device, so that the operation of the image forming apparatus cannot be continued in some cases. Even if it does not stop the operation of the image forming apparatus, such aggregation of toner particles in a toner-housing container prevents replenishing of toner particles, reducing the toner density to form abnormal images potentially. Suppressing the occurrence of blocking is improving the blocking resistance of toners. In addition, lowering the glass transition temperature of a toner binder also impairs storageability of the toner on the surface of a fixed image. This fixed image tends to melt and be transferred and thus it is attached onto another recording medium stacked thereon, so that the formed image cannot be stored for a long period of time.
The glass transition temperature is a key factor in designing a toner binder. The method of simply lowering the glass transition temperature could not produce a toner fixable by a fixing device set to a lower temperature than before.
Meanwhile, using a crystalline resin as a toner binder has long been known as a method for making blocking resistance and low-temperature fixing property both satisfactory. However, use of the crystalline resin raises a problem that the obtained toner is insufficient in elasticity upon melting to cause hot offset.
As a method for making blocking resistance and low-temperature fixing property both satisfactory, there has been proposed a core-shell toner containing a shell obtained using the melting suspending method or the emulsifying aggregating method (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2007-70621 and 2004-191927). These techniques are still insufficient in order to achieve good blocking resistance while keeping low-temperature fixing property.
Furthermore, a method focusing on a crystalline resin to solve the above problems has been proposed (see JP-A No. 2010-217849). This method has problems that the crystalline resin is susceptible to external conditions (e.g., heat history and/or partial mixing of phases during production, storing and fixing) so that its crystalline structure is not stabilized, which adversely affects various properties of the obtained toner, blocking resistance and stability of the formed image.